1. Field of the Invention
The present invention relates generally to cordless telephones and, more particularly, is directed to a cordless telephone of so-called multi-channel access (MCA) type in which a master or base station includes a plurality of sets of transmitting and receiving circuits and communication channels can be simultaneously opened between the master station and a plurality of remote stations or handset units.
2. Description of the Prior Art
Generally, the range of movement permitted the user of a telephone is limited by the length of the telephone cord and practical considerations place a limit on such cord length. To overcome this problem, cordless telephones are coming into more frequent use in this and certain other countries.
Typically, as shown in FIG. 1, such a cordless telephone system consists of a transmitter-receiver 1, in the form of a cordless handset, and a transmitter-receiver 2 forming a so-called base unit that is physically coupled or connected to a telephone network line 3. The handset 1 and the base unit 2 are coupled with each other through radio waves transmitted and received by respective antennas 100 and 200.
The useful distance between the handset 1 and the base unit 2 is referred to as the service area and is typically no more than about 100 meters, with a maximum transmission range being set by the relevant governmental agencies. In Japan, for example, the frequencies of the radio waves transmitted between the handset 1 and the base unit 2 are typically in a 250 MHz band in an up-channel and in a 380 MHz band in a down-channel, and 87 duplex channels are permitted for each system of the MCA type.
In using a cordless telephone, the telephone subscriber can initiate an outgoing call and receive an incoming call through the handset 1 in a manner similar to that used with a standard telephone system, and the telephone subscriber can move about freely with the handset without the annoyance of tangling a lengthy telephone cord.
As shown in FIG. 2, in a proposed cordless telephone disclosed in Japanese Patent Application No. 63-68725, the handset unit 1 may generally include a transmitting circuit 110 and a receiving circuit 120. The transmitting circuit 110 converts an audio signal St received from a mouthpiece or microphone 111 and a command signal CMND, which is hereinafter described more fully, to an up channel frequency-modulated (FM) signal Su and transmits the same from the antenna 100. The receiving circuit 120 is adapted to receive a down channel FM signal Sd from the antenna 100 and to demodulate the same to provide an audio signal Sr and a command signal CMND.
A dial key pad 131 and a talk key 132 are connected to a microcomputer 140 which will be hereinafter further described. Each time the talk key 132 is depressed, the operating mode of the handset unit 1 is alternately changed over between a standby mode and a talk mode. When in its standby mode, the handset unit 1 is adapted only for reception of the down channel signal Sd. Conversely, when in its talk mode, the handset unit 1 is adapted for transmission and reception of the up channel and down channel signals Su and Sd.
An oscillator 138 may be energized in response to a signal from the microcomputer 140 for providing an oscillating signal to a speaker 139 so that the latter emits a ringing bell sound.
The microcomputer 140 is further shown to be connected to the transmitting circuit 110 and the receiving circuit 120 and is operative to generate the command signal CMND which is transmitted through the transmitting circuit 110 as part of the up channel signal Su, and also to determine the status or nature of the command signal CMND derived from the receiving circuit 120 in response to the down channel signal Sd. A read only memory 141 is connected with the microcomputer 140 for providing to the latter an identification code ID which identifies the respective handset 1.
The base unit 2 is shown on FIG. 2 to be provided with a transmitting circuit 210 and a receiving circuit 220 generally corresponding to the transmitting circuit 110 and the receiving circuit 120 of the handset unit 1. The receiving circuit 220 is set in the standby mode of the up control channel when a telephone call is not in progress.
Further, the base unit 2 is provided with a four-line to two-line converting circuit 231, a switch circuit 232 corresponding to a hook switch of an ordinary or standard telephone set, a signal forming circuit 233 for generating a dial tone or tone encode signal TENC, which may be a DTMF or dual tone multi-frequency signal, and a microcomputer 240 for controlling the overall operation of the cordless telephone. The microcomputer 240 is connected with a read only memory (ROM) 241 for storing an identification code ID corresponding to that stored in the memory 141 of the associated handset.
As shown in FIG. 3, in a suitable format for the command signal CMND, there are provided, at the head thereof, a bit synchronizing signal BSYN of more than 12 bits, in this example, 24 bits, and subsequent thereto, a frame synchronizing signal FSYN of 16 bits. These signals BSYN and FSYN have particular bit patterns, for example, as follows:
BSYN is "010101010101010101010101" for all channels;
FSYN is "1100010011010110" for the up channels; and
FSYN is "1001001100110110" for the down channels.
The command signal CMND is further shown to include, subsequent to the signal FSYN, a 25-bit identification code ID, a 15-bit error correcting code ECC for the code ID and a 5-byte control code CTRL, arranged in that order.
The control code CTRL includes a code by which the communication channel between the handset unit 1 and the base unit 2 is controlled to open and/or close, a code indicating the transmission of a telephone number of the party to be called, and a code indicating the telephone number of a corresponding telephone apparatus. For example, when the communication channel is opened, the control code CTRL includes data of a first bit pattern and dummy data, whereas, when the control code CTRL indicates the telephone number of the party to be called, the control code CTRL includes data of a second bit pattern and data of a corresponding telephone number. In other words, the control code CTRL includes data (operation code) indicating the kind of control operation to be performed and data (operand) necessary for such control operation.
When the command signal CMND from the base unit 2 is received by the handset unit 1, it is determined by the microcomputer 140 whether or not the identification code ID included in the received command signal CMND is identical with the identification code ID stored in the ROM 141 of the handset unit 1. When the corresponding codes are identical with each other, it is determined that the received command signal CMND is valid. If not, it is determined that the command signal CMND is invalid.
Similarly, when a valid command signal CMND is received by the base unit 2 from the associated handset unit 1, the base unit 2 transmits to the handset unit 1 a command signal CMND unit 2 transmits to the handset unit 1 a command signal CMND including the same identification code ID as was included in the command signal CMND received from the handset unit 1.
In making an outgoing call and in receiving an incoming call, the microcomputers 140 and 240 are operated as follows:
In making an outgoing call from the handset unit 1, depression of the talk key 132 when, for example, the handset unit 1 is in the standby mode, is detected by the microcomputer 140 which supplies a transmission control signal TCTL to the transmitting circuit 110. In response thereto, the transmitting circuit 110 is enabled to transmit an up channel signal Su through the up control channel. Further, the microcomputer 140 forms a command signal CMND in which the control code CTRL indicates the request for an outgoing call. Such command signal CMND is supplied from the microcomputer 140 to the transmitting circuit 110 in which it is converted to the up control channel FM (frequency-modulated) signal Su, and this signal Su is transmitted from the antenna 100 for reception by the antenna 200 of the base unit I.
In the base unit 2, the command signal CMND is derived from the receiving circuit 220 of the base unit 2, and is supplied to the microcomputer 240, which determines therefrom that an outgoing call is being made. Subsequently, the microcomputer 240 causes the switch circuit 232 to be placed in the ON-hook or closed state, thereby connecting the converting circuit 231 through the switch circuit 232 to the telephone network line 3. Simultaneously, the transmitting circuit 210 is enabled by the transmitting control signal TCTL from the microcomputer 240 to transmit the signal Sd of the down control channel. The microcomputer 240 forms the command signal CMND which includes an identification code ID the same as the received identification code ID and a control code CTRL indicating permission to proceed with the outgoing call and the channel number of the communication channel. This command signal CMND is converted to the FM signal Sd in the transmitting circuit 210 and is transmitted through the antenna 200 to the antenna 100 of the handset unit 1.
Then, the receiving circuit 120 of the handset unit 1 derives the command signal CMND from the received down channel signal Sd. This command signal CMND is supplied to the microcomputer 140 which determines therefrom whether or not the identification code ID included in the received command signal CMND is identical with the identification code ID stored in the ROM 141. In the event that the two identification codes are identical with each other, the microcomputer 140 provides the control signals TCTL and RCTL by which the transmitting circuit 110 and the receiving circuit 120 are set in the talk mode through the communication channel identified by the channel number indicated in the control signal CTRL included in the received command signal CMND.
When the communication channel of the handset 1 is thus appointed, the microcomputer 240 supplies the transmitting circuit 210 and the receiving circuit 220 with respective transmitting and receiving control signals TCTL and RCTL, respectively. In response to such control signals TCTL and RCTL from the microcomputer 240, the transmitting circuit 210 and the receiving circuit 220 are set in the talk mode through the communication channel identified by the channel number which the microcomputer 240 in the base unit 2 appoints for the handset unit 1. Accordingly a communication channel is opened between the handset unit 1 and the base unit 2.
When the user inputs the telephone number for the outgoing call by manipulation of the dial key pad 131 of the handset unit 1, the microcomputer 140 forms the command signal CMND in which the control signal CTRL indicates the transmission mode of the telephone number and the telephone number to be called. This command signal CMND is converted by the circuit 110 to the FM signal Su of the communication channel appointed by the base station 2 and such signal Su is transmitted from the antenna 100.
In the base unit 2, the signal Su is received by the antenna 200, and the receiving circuit 220 of the base unit 2 generates the command signal CMND from the received FM signal Su. The control code CTRL of such command signal CMND indicates the transmission mode of the telephone number and the telephone number to be called and, in response thereto, the microcomputer 240 controls the signal forming circuit 233 so as to form a tone encode signal TENC corresponding to the telephone number transmitted from the handset unit 1 and which may be in the form of a dual-tone multi-frequency (DTMF) signal. This tone encode signal TENC is supplied through the converting circuit 231 and the switch circuit 232 to the telephone network line 3. When the called party comes on the line in response to the reception of the tone encode signal TENC, telephone communication becomes possible.
When the telephone conversation is ended, the talk key 132 of the handset unit 1 is released from the talk mode or opened, and the microcomputer 140 responds thereto by providing to the transmitting circuit 110 the command signal CMND whose control code CTRL indicates the end of the telephone conversation. Such command signal CMND is transmitted to the base station 2 in the form of the FM signal Su, and then the transmission of the FM signal Su is stopped by the transmission control signal TCTL from the microcomputer 140.
In response to the transmission to the base unit 2 of the command signal whose control code CTRL indicates the completion of a telephone conversation, the microcomputer 240 controls the switch circuit 232 to return the same to its ON-hook or open state. Further, the microcomputer 240 responds to the corresponding command signal CMND from the receiving circuit 220 by providing a signal TCTL to the transmitting circuit 210 by which transmission of the FM signal Sd is stopped. Accordingly, the base unit 2 is placed in the standby mode for awaiting an incoming call.
When an incoming call is received through the telephone network line 3, the ring tone signal thereof is detected by a detecting circuit 234, and the detected signal is supplied to the microcomputer 240. The microcomputer 240 responds to the detected ring tone signal to form the command signal CMND whose control code indicates the existence of the incoming call and the channel number of the communication channel. This command signal CMND is supplied to the transmitting circuit 210, and the corresponding FM signal Sd is transmitted to the handset unit 1 through the down control channel.
In the handset unit 1, the receiving circuit 220 generates a command signal CMND corresponding to that included in the transmitted signal Sd, and this command signal CMND is supplied to the microcomputer 140. Then, the microcomputer 140 determines whether or not the identification code ID included in such command signal CMND is identical with the identification code ID stored in the ROM 141. If the identification codes are identical, the microcomputer 140 controls the oscillating circuit 138 to provide a ring signal. This ring signal from the oscillating circuit 138 is supplied to the speaker 139 from which a bell sound emanates for indicating an incoming call.
When the talk key 132 is changed from the standby mode to the talk mode, processing similar to that described above for making an outgoing call is performed, whereby the handset unit 1 is connected to the telephone network line 3 via the communication channel and the base unit 2. Therefore, telephone communication with the calling party can be carried out.
When the telephone conversation is ended, the talk key 132 is changed from the talk mode to the standby mode, whereby the microcomputer 140 generates a command signal CMND for indicating the termination of the telephone conversation. This command signal CMND is transmitted in the form of the FM signal Su, from which the receiving circuit 220 derives a corresponding command signal CMND indicating to the microcomputer 240 that the conversation is ended, and then further transmission of the FM signal Su is inhibited by the transmission control signal TCTL from the microcomputer 140. The microcomputer 240 responds to the indication that the conversation is ended by providing control signals tCTL and RCTL by which signal transmission by the transmitting circuit 210 in the base unit 2 is inhibited and the receiving circuit 220 is changed to its standby mode for the up control channel by the control signal RCTL. Further, the switch circuit 232 is changed to its ON-hook state. Therefore, the handset unit 1 and the base unit 2 are placed in the standby mode for making an outgoing call or for receiving an incoming call.
Although the above-described cordless telephone system permits the user to move over a relatively wide area away from the base unit 2 while either making or receiving a call by means of the handset unit I, the use of such cordless telephone system is otherwise undesirably limited by the provision of only a single handset unit in association with a single base unit. Accordingly, a so-called multi-cordless telephone system has been proposed in which a plurality of handset units are associated with a single base unit. In this proposed multi-cordless telephone system, all of the handset units have a common identification code ID, so that any one of the handset units can communicate with the single base unit, but, when one handset unit is thus operated, the other handset units cannot be used.
In order to avoid the above problems, it has been proposed to provide a multi-cordless telephone system in which a plurality of handset units are associated with a plurality of base units located at a common base station and adapted for connection to respective telephone network lines. However, this proposal is disadvantageous in that, when any one of the handset units is operated to make an outgoing call, all of the base units simultaneously respond to the resultant transmission from the operated handset unit.